A method which has been used to increase the capacity of cellular communication systems depends upon the concept of hierarchical cells wherein a macro-cell layer is underlayed by a layer of typically smaller cells having coverage areas within the coverage area of the macro-cell. In this way, the smaller cells, known as micro-cells, pico-cells, or femto-cells (hereinafter collectively referred to as femto-cells) are located within the same coverage area as larger overlaying macro cells. The femto-cells have much smaller coverage thereby allowing a much closer reuse of resources. Frequently, the macro-cells are used to provide coverage over a large area, and the smaller underlay cells are used to provide additional capacity in densely populated areas and hotspots, for example. Furthermore, femto-cells can also be used to provide coverage in specific locations such as within a residential home or office. In order to efficiently exploit the additional resources, it is important that any interference between cells is minimized.
Currently 3rd generation cellular communication systems based on code-division multiple access (“CDMA”) technology, such as the Universal Mobile Telecommunication System (“UMTS”) or 4G Long Term Evolution (“LTE”), are being deployed with a trend towards introducing a large number of femto-cells. For example, it is envisaged that a Home-evolved NodeB (“HeNB”) may be deployed having a target coverage area of only a single residential dwelling. A widespread introduction of such a system could result in a very large number of small underlay cells within a single macro-cell.
Generally, during deployment of a cellular network of macro cells and underlay cells, it would be desirable for the planned layout to be fixed and known. In a centrally controlled database deployment process, well established techniques exist to calculate the optimal pilot power levels for fixed and known layouts. For example, the underlay cells of such systems can receive a neighbor list identifying a number of neighbor cells and the underlay cells' measured pilot signal power levels of these neighbor cells. These levels for each neighbor macro-cell can be measured and reported back to the central database, such as in a radio network controller (“RNC”) or a Mobile Switching Centre (“MSC”). The central database could then use these measurements to determine an appropriate pilot power level for that underlay-cell. However, a problem arises in the ad hoc introduction of underlaying femto-cells, such as residential deployments of femto-cell HeNBs where the deployment process is incremental, unilateral, and changeable.
In particular, without coordination between the RNC of the macro layer and the HeNB layer, and considering a much smaller dynamic range of power control at the HeNB, there are cases where the macro layer can suffer interference from the control pilot channel power of the HeNB in close proximity thereto and vice versa. Different from macro eNBs, femto-cell HeNBs typically only support a small number of pre-registered user equipment (UE). To reduce interference towards the macro layer, it is desirable that HeNBs employ minimum necessary power so as to cover all pre-registered UEs that are within the residential area. Although this minimum power level can be determined easily for connected-mode UEs according to the dedicated signalling between the HeNB and the connected-mode UEs, it is difficult to obtain the minimum power level to ensure coverage for idle-mode UEs as no dedicated signalling exists between the HeNB and the idle-mode UEs. Additionally, an ad hoc cellular system that consists of femto-cells of HeNBs and macro-layer mobile user equipment is by definition changeable. No settings, regardless of how optimal, are stable for long, and they will change over time.